Optical pickup device

ABSTRACT

A pickup device includes a diffraction grating  12  for separating a light beam emitted from the light source into at least three light beams. The diffraction grating  12  is divided into four regions by straight lines extending in a direction parallel to a tangential direction of tracks of an optical information recording medium. A periodic structure of a second region  12 B has a phase difference of approximately 180 degrees from a periodic structure of a third region  12 C, and a periodic structure of a first region  12 A has a phase difference of approximately 180 degrees from a periodic structure of a fourth region  12 D.

TECHNICAL FIELD

The invention relates to an optical pickup device that is used in anoptical information processor for performing processing such asrecording of information onto an optical information recording mediumand playback or erasure of information recorded on the opticalinformation recording medium.

BACKGROUND ART

Reading recorded information from an optical information recordingmedium (optical disc) such as a CD (Compact Disc) and a DVD (DigitalVersatile Disc) is conducted by converging a light beam emitted from alight source such as a semiconductor laser device on a recording trackof the optical disc by using an objective lens and converting reflectedlight from the optical disc to an electric signal by a photodetector. Inorder to accurately converge a light beam on a desired recording trackof a rapidly spinning optical disc, a focus error signal and a trackingerror signal are detected and the position of the objective lens iscontrolled according to surface displacement, eccentricity, and the likeof the optical disc.

A differential push-pull (DPP) method is known as a typical method fordetecting a tracking error signal. In the DPP method, a light beam isseparated into three beams: a main beam; a +1^(st) order diffractedbeam; and a −1^(st) order diffracted beam. These three beams arerespectively converged on three adjacent guide grooves formed at aprescribed pitch on the optical disc. Push-pull signals respectivelyobtained by detecting reflected light of the three beams and performingan arithmetic operation have a phase difference of 180 degrees betweenthe main beam and the +1^(st) and −1^(st) order diffracted beams.Therefore, by performing arithmetic processing of each push-pull signal,only offset components included in the push-pull signals are selectivelycancelled each other, whereby an excellent tracking error signal can bedetected. Accordingly, the DDP method has been widely used especially ina DVD recording optical pickup (e.g., see Patent document 1).

There are various standards for currently used optical discs, and aguide groove pitch varies depending on the standards of the opticaldiscs. For example, optical discs such as a write once type DVD-R(Recordable) and an erasable type DVD-RW (Disk ReWritable) have a guidegroove pitch of 0.74 μm, and optical discs such as an erasable typeDVD-RAM (Random Access Memory) has a guide groove pitch of 1.23 μm. Anoptical pickup device that enables recording and playback of two or moretypes of optical discs of different standards has been demanded. Thefollowing optical pickup device is proposed in view of this demand(e.g., see Patent document 2).

In the optical pickup device disclosed in Patent document 2, a specialdiffraction grating for separating a light beam is divided into threeregions, and the phase of grating grooves periodically provided in eachregion is sequentially shifted by 90 degrees. A tracking error detectionmethod using such a special diffraction grating is called an in-line DPPmethod, and the in-line DPP method enables stable tracking errordetection on a plurality of optical information recording media havingdifferent guide groove pitches.

Patent document 1: Japanese Patent Publication for Opposition No.4-34212

Patent document 2: Japanese Laid-Open Patent Publication No. 2004-145915

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, a conventional optical pickup device using the conventionalin-line DPP method has the following problems.

FIG. 18 shows convergence spots of light beams that are converged on anoptical information recording medium by a conventional optical pickupdevice. A convergence spot 101 corresponding to a +1^(st) orderdiffracted beam has higher intensity on the right side in a radialdirection X of the optical information recording medium and has lowerintensity on the left side. On the other hand, a convergence spot 102corresponding to a −1^(st) order diffracted beam has lower intensity onthe right side and has higher intensity on the left side. This can beexplained as follows:

As shown in FIG. 19, in a special diffraction grating used in theconventional in-line DPP method, the phase of grating grooves 119 a in aregion 119 is ahead of that of grating grooves 120 a in a central region120 by 90 degrees, and the phase of grating grooves 121 a in a region121 is behind that of the grating grooves 120 a in the central region120 by 90 degrees. Accordingly, the phase of the +1^(st) orderdiffracted beam that has passed through the region 119 is ahead of thatof the +1^(st) order diffracted beam that has passed through the centralregion 120 by 90 degrees, and the phase of the +1^(st) order diffractedbeam that has passed through the region 121 is behind that of the+1^(st) order diffracted beam that has passed through the central region120 by 90 degrees. The phase relation of the grating grooves anddiffracted beams is opposite for the −1^(st) order diffracted beam. Inother words, the phase of the −1^(st) order diffracted beam that haspassed through the region 119 is behind that of the −1^(st) orderdiffracted beam that has passed through the central region 120 by 90degrees, and the phase of the −1st order diffracted beam that has passedthrough the region 121 is ahead of that of the −1^(st) order diffractedbeam that has passed through the central region 120 by 90 degrees.

Accordingly, the +1^(st) order diffracted beam has larger intensitydistribution on the region 121 side where the phase is retarded, and theconvergence spot 101 corresponding to the +1^(st) order diffracted beamon the optical information recording medium has higher intensity on theright side and lower intensity on the left side. On the other hand, the−1^(st) order diffracted beam has larger intensity distribution on theregion 119 side where the phase is retarded, and the convergence spot102 corresponding to the −1^(st) order diffracted beam has lower lightintensity on the right side and higher intensity on the left side.

FIG. 20 shows signal waveforms of a DPP signal obtained by the aboveconvergence spots. In FIG. 20, the ordinate indicates signal strengthand the abscissa indicates a relative position of the convergence spoton the optical information recording medium. MPP is a push-pull signalof a main beam corresponding to a 0^(th) order diffracted beam, SPP1 isa push-pull signal of a preceding sub-beam corresponding to a +1^(st)order diffracted beam, SPP2 is a push-pull signal of a succeedingsub-beam corresponding to a −1st order diffracted beam, and DPP is atracking error signal (differential push-pull signal) obtained from MPP,SPP1, and SPP2 as shown by the formula (1):

DPP=MPP−k×(SPP1+SPP2)  (1)

where k is an arbitrary amplification factor. Since the respectiveconvergence spots corresponding to the +1^(st) order diffracted beam andthe −1^(st) order diffracted beam have left-right asymmetric intensitydistribution, the phase difference of SSP1 and SSP2 from MPP is shiftedfrom 180 degrees. If there is a signal strength difference between SPP1and SPP2, DPP is shifted from a proper value and therefore eachconvergence spot cannot be formed on the same guide groove, hinderingstable tracking error signal detection by the in-line DPP method. Notethat, the positions where SPP1, SPP2, and DPP have proper values areshown by chain line in FIG. 20.

The invention is made to solve the above conventional problems and it isan object of the invention to implement an optical pickup device forconducting stable tracking error detection on a plurality of opticalinformation recording media having different guide groove pitches whilemaintaining the advantages of the in-line DPP method.

Means for Solving the Problems

In order to achieve the above object, an optical pickup device of theinvention includes a diffraction grating that is divided into fourregions having different phases from each other.

More specifically, an optical pickup device according to the inventionis an optical pickup device for recording information onto an opticalinformation recording medium and reading and erasing informationrecorded on the optical information recording medium. The optical pickupdevice includes: a light source; a diffraction grating for separating alight beam emitted from the light source into at least three lightbeams; and a photodetector for receiving the separated light beamsreflected from the optical information recording medium. The diffractiongrating is divided into a first region, a second region, a third region,and a fourth region having periodic structures of different phases bydividing lines extending in a direction parallel to a tangentialdirection of tracks of the optical information recording medium. Thesecond region and the third region are located between the first regionand the fourth region sequentially from the first region side. Theperiodic structure of the second region has a phase difference ofapproximately 180 degrees from the periodic structure of the thirdregion, and the periodic structure of the first region has a phasedifference of approximately 180 degrees from the periodic structure ofthe fourth region.

The optical pickup device of the invention includes such a diffractiongrating that the periodic structure of the second region has a phasedifference of approximately 180 degrees from the periodic structure ofthe third region and the periodic structure of the first region has aphase difference of approximately 180 degrees from the periodicstructure of the fourth region. Therefore, the phase of a +1^(st) orderdiffracted beam that has passed through the first region is advancedwith respect to that of the +1^(st) order diffracted beam that haspassed through the second region. The phase of the +1^(st) orderdiffracted beam that has passed through the fourth region is advancedwith respect to that of the +1^(st) order diffracted beam that haspassed through the third region. The phase of the −1^(st) orderdiffracted beam, on the other hand, is retarded in both cases.Accordingly, unlike the conventional in-line DPP method, spot shapes donot become left-right asymmetric, but the intensity distribution ofconvergence spots becomes left-right asymmetric with respect to anextending direction of guide grooves. An optical pickup device forconducting stable tracking error detection on a plurality of opticalinformation recording media having different guide groove pitches canthus be implemented.

In the optical pickup device of the invention, a distance between thedividing line dividing the first region and the second region from eachother and the dividing line dividing the second region and the thirdregion from each other is preferably equal to a distance between thedividing line dividing the second region and the third region from eachother and the dividing line dividing the third region and the fourthregion from each other.

In the optical pickup device of the invention, the light beamspreferably include a 0^(th) order diffracted beam, a +1^(st) orderdiffracted beam, and a −1^(st) order diffracted beam.

In the optical pickup device of the invention, it is preferable that aplurality of guide grooves are periodically formed on a recordingsurface of the optical information recording medium, and each of thelight beams is converged on one of the plurality of guide grooves.

Preferably, the optical pickup device of the invention further includesan arithmetic processing circuit for detecting a tracking error signalby a differential push-pull method based on an output signal of thephotodetector.

In the optical pickup device of the invention, it is preferable that thephotodetector includes at least three light receiving elementsrespectively corresponding to the reflected light beams, and each of thelight receiving elements is divided into a plurality of light receivingregions.

In the optical pickup device of the invention, a center of the lightbeam emitted from the light source is preferably positioned in thesecond region or the third region.

In the optical pickup device of the invention, it is preferable that thelight source includes a first light source and a second light source,and a straight line connecting a center of a light beam emitted from thefirst light source and a center of a light beam emitted from the secondlight source crosses at least one of the dividing line dividing thefirst region and the second region from each other, the dividing linedividing the second region and the third region from each other, and thedividing line dividing the third region and the fourth region from eachother.

In the optical pickup device of the invention, it is preferable that thelight source includes a first light source and a second light source,and a straight line connecting a center of a light beam emitted from thefirst light source and a center of a light beam emitted from the secondlight source crosses the dividing line dividing the second region andthe third region from each other.

In the optical pickup device of the invention, it is preferable that thelight source includes a plurality of light sources, and a center of atleast one of light beams respectively emitted from the plurality oflight sources is positioned in the second region or the third region.

In the optical pickup device of the invention, the periodic structure ofthe first region of the diffraction grating preferably has a phasedifference of 10 degrees to 350 degrees from the periodic structure ofthe second region. More preferably, the periodic structure of the firstregion of the diffraction grating has a phase difference ofapproximately 90 degrees from the periodic structure of the secondregion.

Preferably, the optical pickup device of the invention further includesan objective lens for converging the at least three light beams onto arecording surface of the optical information recording medium asindependent convergence spots, and a region of the diffraction gratingon which a range of the emitted light beam corresponding to an effectivebeam diameter determined by an aperture diameter of the objective lensis incident is a region including the first region, the second region,the third region, and the fourth region.

In this case, a sum of a width of the second region and a width of thethird region is preferably in a range of 10% to 40% of the effectivebeam diameter.

EFFECTS OF THE INVENTION

The invention can thus implement an optical pickup device for conductingstable tracking error detection on a plurality of optical informationrecording media having different guide groove pitches while maintainingthe advantages of the in-line DPP method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical pickup device according to anembodiment of the invention;

FIG. 2 is a circuit diagram of a photodetector of the optical pickupdevice according to the embodiment of the invention;

FIG. 3 is a plan view of a diffraction grating of the optical pickupdevice according to the embodiment of the invention;

FIG. 4 is a plan view showing the shapes of convergence spots formed ona recording surface of an optical information recording medium by theoptical pickup device according to the embodiment of the invention;

FIG. 5 is a graph showing signal waveforms obtained by the opticalpickup device according to the embodiment of the invention;

FIG. 6 is a plan view showing an example of a positional relationbetween the diffraction grating of the optical pickup device accordingto the embodiment of the invention and the respective centers of lightbeams;

FIG. 7 is a plan view showing an example of a positional relationbetween the diffraction grating of the optical pickup device accordingto the embodiment of the invention and the respective centers of lightbeams;

FIG. 8 is a plan view showing an example of a positional relationbetween the diffraction grating of the optical pickup device accordingto the embodiment of the invention and the respective centers of lightbeams;

FIG. 9 is a plan view showing an example of a positional relationbetween the diffraction grating of the optical pickup device accordingto the embodiment of the invention and the respective centers of lightbeams;

FIG. 10 is a plan view showing an example of a positional relationbetween the diffraction grating of the optical pickup device accordingto the embodiment of the invention and the respective centers of lightbeams;

FIG. 11 is a plan view showing a modification of the diffraction gratingof the optical pickup device according to the embodiment of theinvention;

FIG. 12 is a plan view showing a modification of the diffraction gratingof the optical pickup device according to the embodiment of theinvention;

FIG. 13 is a plan view showing a modification of the diffraction gratingof the optical pickup device according to the embodiment of theinvention;

FIGS. 14A through 14C are graphs showing relations between the shiftamount of an objective lens and variation in differential push-pullsignal amplitude in the optical pickup device of the embodiment of theinvention, wherein FIGS. 14A through 14C respectively show the casewhere the phase difference between first and second regions is 0degrees, 90 degrees, and 180 degrees;

FIG. 15 is a graph showing a relation between the phase differencebetween first and second regions of the diffraction grating of theoptical pickup device according to the embodiment of the invention andthe change rate of differential push-pull signal amplitude;

FIG. 16 is a graph showing a relation between the width of second andthird regions of the diffraction grating of the optical pickup deviceaccording to the embodiment of the invention and the change rate ofdifferential push-pull signal amplitude;

FIG. 17 is a graph showing a relation between the width of the secondand third regions of the diffraction grating of the optical pickupdevice according to the embodiment of the invention and the push-pullsignal amplitude;

FIG. 18 is a plan view showing the shapes of convergence spots formed ona recording surface of an optical information recording medium by aconventional optical pickup device;

FIG. 19 is a plan view of a diffraction grating of the conventionaloptical pickup device; and

FIG. 20 is a graph showing signal waveforms obtained by the conventionaloptical pickup device.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   11 light source    -   12 diffraction grating    -   12A first region    -   12B second region    -   12C third region    -   12D fourth region    -   12 a grating groove    -   12 b grating groove    -   12 c grating groove    -   12 d grating groove    -   15 half mirror    -   16 photodetector    -   17 integrated circuit board    -   18 collimating lens    -   19 objective lens    -   21A light receiving element    -   21B light receiving element    -   21C light receiving element    -   23 arithmetic processing circuit    -   24 subtracter    -   25 subtracter    -   26 subtracter    -   27 adder    -   28 amplifier    -   29 subtracter    -   31 light beam    -   31 a main beam

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be described with reference to theaccompanying drawings. FIG. 1 schematically shows a structure of anoptical pickup device according to an embodiment of the invention.

As shown in FIG. 1, the optical pickup device of the embodiment includesa light source 11, such as a semiconductor laser element, for emitting alight beam 31 for performing recording of information onto an opticalinformation recording medium 51 and playback of information recorded onthe optical information recording medium 51, a diffraction grating 12for diffracting and separating the light beam 31 into at least threelight beams (not shown): a main beam that is a 0^(th) order diffractedbeam; a sub-beam that is a +1^(st) order diffracted beam; and a sub-beamthat is a −1^(st) order diffracted beam, a half mirror 15 for guidingthe separated light beams 31 to the optical information recording medium51, and an integrated circuit board 17 having a photodetector 16 forreceiving the light beams 31 reflected from the optical informationrecording medium 51.

A collimating lens 18 and an objective lens 19 are placed between thehalf mirror 15 and the optical information recording medium 51. Thelight beam 31 emitted from the light source 11 is first diffracted andseparated by the diffraction grating 12 into at least three light beams:a 0^(th) order diffracted beam; a +1^(st) order diffracted beam; and a−1^(st) order diffracted beam. These beams are then reflected by thehalf mirror 15 and reach the objective lens 19 through the collimatinglens 18. The 0^(th) order diffracted beam, the +1^(st) order diffractedbeam, and the −1^(st) order diffracted beam thus obtained by thediffraction grating 1 are then independently converged on a recordingsurface of the optical information recording medium 51 by the objectivelens 19 to form three convergence spots.

FIG. 2 shows a circuit structure of the integrated circuit board 17having the photodetector 16 in the optical pickup device of FIG. 1. Asshown in FIG. 2, the integrated circuit board 17 has light receivingelements 21A, 21B, and 21C and an arithmetic processing circuit 23 forperforming an arithmetic operation of signals from the light receivingelements. A main beam 31 a and two sub-beams 31 b and 31 c separatedfrom the emitted light beam 31 by the diffraction grating 12 arereceived by the light receiving elements 21A, 21B, and 21C,respectively. Each of the light receiving elements 21A, 21B, and 21C isdivided into a plurality of light receiving regions.

Signals detected by the light receiving elements 21A, 21B, and 21C areapplied to the arithmetic processing circuit 23. The arithmeticprocessing circuit 23 has subtracters 24, 25, and 26 for receivingsignals from the light receiving elements 21A, 21B, and 21C,respectively, and an adder 27, an amplifier 28, and a subtracter 29 forreceiving outputs from the subtracters 24, 25, and 26. The subtracters24, 25, and 26 receive signals from the light receiving elements 21A,21B, and 21C and output push-pull signals MPP, SPP1, and SPP2,respectively. The adder 27, the amplifier 28, and the subtracter 29 ofthe arithmetic processing circuit 23 will be described later.

In the circuit structure of FIG. 2, each light receiving element isdivided into two light receiving regions. However, each light receivingelement may be divided into three or more light receiving regions. InFIG. 2, each beam on each light receiving element is schematically shownto have a circular shape. However, the beam shape is not limited tothis. When an astigmatic method is used for a focus error signal, eachbeam on each light receiving element is rotated by approximately 90degrees. Each light receiving element therefore needs to be rotated by90 degrees in advance in this case.

The optical pickup device of this embodiment is characterized in thediffraction grating 12 for diffracting the light beam 31 emitted fromthe light source 11, and is characterized especially in a periodicstructure of the diffraction grating 12. FIG. 3 shows a periodicstructure, that is, a grating pattern, of the diffraction grating 12.

As shown in FIG. 3, a grating surface of the diffraction grating 12 isdivided into four regions: a first region 12A; a second region 12B; athird region 12C; and a fourth region 12D by three dividing lines D1,D2, and D3 extending in an extending direction of guide grooves of theoptical information recording medium 51 (hereinafter, referred to as Ydirection), that is, in a direction substantially parallel to atangential direction of tracks of the optical information recordingmedium 51. Note that, in this case, the parallel direction means aparallel direction in view of an optical system provided between thediffraction grating 12 and the optical information recording medium 51.The first region 12A and the second region 12B are adjacent to eachother with the dividing line D1 interposed therebetween. The secondregion 12B and the third region 12C are adjacent to each other with thedividing line D2 interposed therebetween. The third region 12C and thefourth region 12D are adjacent to each other with the dividing line D3interposed therebetween.

As shown in FIG. 3, the first region 12A, the second region 12B, thethird region 12C, and the fourth region 12D have grating grooves 12 a,12 b, c, and 12 d periodically provided along a radial direction of theoptical information recording medium 51 (hereinafter, referred to as Xdirection), respectively. Note that, in this embodiment, the gratinggrooves 12 a, 12 b, 12 c, and 12 d have the same width, and the portions(protruding portions) between the grating grooves 12 a, 12 b, 12 c, and12 d have the same width.

The phase of the periodic structure formed by the grating grooves 12 ain the first region 12A is substantially 90 degrees ahead of that of theperiodic structure formed in the second region 12B (a phase differenceof substantially +90 degrees). In other words, the arrangement cycle ofthe grating grooves 12 a in the first region 12A is shifted by quartercycle from the arrangement cycle of the grating grooves 12 b in thesecond region 12B in +Y direction. The phase of the periodic structureformed in the fourth region 12D is substantially 90 degrees behind thatof the periodic structure formed in the second region 12B (a phasedifference of substantially −90 degrees). In other words, thearrangement cycle of the grating grooves 12 d in the fourth region 12Dis shifted by quarter cycle from the arrangement cycle of the gratinggrooves 12 b in the second region 12B in −Y direction. Accordingly, theperiodic structure in the first region 12A has a phase difference ofsubstantially 180 degrees from the periodic structure in the fourthregion 12D. The phase of the periodic structure in the third region 12Cis shifted by substantially 180 degrees from that of the periodicstructure in the second region 12B. In other words, the arrangementcycle of the grating grooves 12 c in the third region 12C is shifted byhalf cycle from the arrangement cycle of the grating grooves 12 b in thesecond region 12B in +Y direction.

Note that the phase difference of the periodic structure between theregions does not have to be exactly 90 degrees or 180 degrees. Since theconvergence spots on the recording surface of the optical informationrecording medium 51 need only have a shape described below, the phasedifference between regions may include an error of about +10 degrees.

In this embodiment, as shown in FIG. 3, the center (the center of alight emitting point) L1 of the light beam emitted from the light source11 is positioned on the dividing line D2 within the assembly accuracyrange of the device.

The emitted light beam incident on the diffraction grating 12 isseparated into a main beam and sub-beams having a prescribed phasedifference by the respective periodic structures formed in the firstregion 12A, the second region 12B, the third region 12C, and the fourthregion 12D. The main beam and the sub-beams are then guided to theoptical information recording medium 51.

Hereinafter, the reason why the optical pickup device of this embodimentcan stably detect tracking errors on optical information recording mediahaving different guide groove pitches will be described.

FIG. 4 shows the shapes of respective convergence spots of the main beam31 a and two sub-beams 31 b and 31 c of the emitted light beam which aregenerated by the diffraction grating 12 on the recording surface of theoptical information recording medium 51. In FIG. 4, X direction showsthe radius direction of the optical information recording medium and Ydirection shows the extending direction of the guide grooves.

In the diffraction grating 12, the diffraction grating in the secondregion 12B has a phase difference of 180 degrees from the diffractiongrating in the third region 12C. Therefore, diffracted light that haspassed through the second region 12B and diffracted light that haspassed through the third region 12C cancel each other, and therespective convergence spots of the sub-beams 31 b and 31 c on therecording surface of the optical information recording medium 51 in FIG.4 have lower intensity in the center. Since the respective convergencespots of the sub-beams 31 b and 31 c need only have lower intensity inthe center, the phase difference between the second region 12B and thethird region 12C may include an error of about +10 degrees from 180degrees.

The phase of the diffraction grating in the first region 12A is 90degrees ahead of that of the diffraction grating in the second region12B. The phase of the diffraction grating in the fourth region 12D is 90degrees ahead of that of the diffraction grating in the third region12C. Accordingly, the phase of the +1^(st) order diffracted beam thathas passed through the first region 12A is advanced by 90 degrees fromthat of the +1^(st) order diffracted beam that has passed through thesecond region 12B. The phase of the +1^(st) order diffracted beam thathas passed through the fourth region 12D is advanced by 90 degrees fromthat of the +1^(st) order diffracted beam that has passed through thethird region 12C. The phase of the −1^(st) order diffracted beam, on theother hand, is retarded by 90 degrees. Accordingly, unlike theconventional in-line DPP method, spot shapes do not become left-rightasymmetric, but the intensity distribution of the convergence spotsbecomes left-right asymmetric with respect to Y direction. In this caseas well, the phase difference between the first region 12A and thesecond region 12B and the phase difference between the fourth region 12Dand the third region 12C may include an error of about ±10 degrees from90 degrees.

As shown in FIG. 4, a plurality of guide grooves 51 a are periodicallyformed on the recording surface of the optical information recordingmedium 51. The respective convergence spots of the main beam 31 a, thesub-beam 31 b, and the sub-beam 31 c of the light beam 31 converged bythe objective lens 19 are located on the same guide groove 51 a, asshown in FIG. 4.

The main beam 31 a, the sub beam 31 b, and the sub beam 31 c arereflected at the respective convergence spots, and the reflected lightbeams corresponding to the respective convergence spots are respectivelyreceived by the light receiving elements 21A, 21B, and 21C of thephotodetector 16. The light receiving elements 21A, 21B, and 21C outputa push-pull signal MPP corresponding to the main beam 31 a, a push-pullsignal SPP1 corresponding to the sub-beam 31 b, and a push-pull signalSPP2 corresponding to the sub-beam 31 c, respectively.

Offset components of the push-pull signals MPP, SPP1, and SPP2 resultingfrom a radial shift of the objective lens 19 (a shift in the radiusdirection of the optical information recording medium) and a tilt of theoptical information recording medium 51 are generated on the same side(the same phase) for each of the radial shift of the objective lens 19and the tilt of the optical information recording medium 51.Accordingly, a differential push-pull (DPP) signal obtained bycancelling the offsets resulting from the radial shift of the objectivelens 19 and the tilt of the optical information recording medium 51 canbe detected by performing an arithmetic operation shown by the formula(2) by using the adder 27, the amplifier 28, and the subtracter 29 shownin FIG. 2:

DPP=MPP−k×(SPP1+SPP2)  (2)

where k is an amplification factor of the amplifier 28.

FIG. 5 shows respective output waveforms of the push-pull signals MPP,SPP1, and SPP2 and the DPP signal obtained by the formula (2). In FIG.5, the ordinate indicates signal strength and the abscissa indicates arelative position of the convergence spot on the optical informationrecording medium 51. As shown in FIG. 5, SPP1 and SPP2 have a phasedifference of exactly 180 degrees from MPP. Even if there is a signalstrength difference between SPP1 and SPP2, the DPP signal obtained bythe formula (2) has a proper value and each convergence spot cantherefore be formed on the same guide groove.

As shown in FIG. 2, the inputs of the adder 27 are respectivelyconnected to the respective outputs of the subtracters 25 and 26, andthe input of the amplifier 28 is connected to the output of the adder27. The inputs of the subtracter 29 are respectively connected to theoutput of the subtracter 24 and the output of the amplifier 28. Thearithmetic operation shown by the formula (2) can be performed with thisstructure. The coefficient k in the formula (2) is used to correct thedifference in light intensity among the main beam 31 a, the sub-beam 31b, and the sub-beam 31 c that are reflected from the optical informationrecording medium 51. When the light intensity ratio of the main beam 31a, the sub-beam 31 c, and the sub-beam 31 c is a:b:b, the coefficient kis a/2b. In other words, the coefficient k is a constant that isdetermined according to the type of the optical information recordingmedium 51. Note that a signal processing circuit having a conventionalstructure may be used in this embodiment.

The optical information recording medium 51 is not limited to a specifictype, and DVDs including a DVD-ROM, a DVD-RAM, a DVD-R, and a DVD-RW andCDs including a CD-ROM, a CD-R, and a CD-RW may be used as the opticalinformation recording medium 51. The wavelength of the light beam 31 canbe determined according to the type of the optical information recordingmedium 51, and is in the range of about 650 nm to about 780 nm in thecase of a DVD and a CD. As for DVDs, stable tracking error signaldetection can be performed on both a DVD having a guide groove pitch of0.74 μm such as a DVD-R and a DVD having a guide groove pitch of 1.23 μmsuch as a DVD-RAM.

In this embodiment, the diffraction grating 12 is placed between thelight source 11 and the half mirror 15 in the optical system shown inFIG. 1. However, the diffraction grating 12 may alternatively be placed,for example, between the half mirror 15 and the collimating lens 18. Theoptical system of FIG. 1 may be replaced with an optical system in whicha light source and a photodetector are integrated (for example, anoptical system that does not use a half mirror), and the diffractiongrating may be placed between the light source and the collimating lens.

In this embodiment, the grating grooves in each region of thediffraction grating 12 are formed along X direction, that is, the radiusdirection of the optical information recording medium. However, thegrating grooves may alternatively be formed in a direction oblique to Xdirection. In this embodiment, the second region 12B and the thirdregion 12C of the diffraction grating 12 have the same width. However,the second region 12B and the third region 12C of the diffractiongrating 12 need not necessarily have the same width.

In this embodiment, a single light beam is emitted from the lightsource. However, the same effects can be obtained even when an opticalpickup device has a plurality of light sources and a plurality of lightbeams are emitted from the light sources.

For example, as shown in FIG. 6, the center L1 of a light emitting pointof a first light source may be positioned on the dividing line D2 andthe center L2 of a light emitting point of a second light source may bepositioned on the dividing line D3 within the assembly accuracy range ofthe device. Alternatively, as shown in FIG. 7, the center L1 of a lightemitting point of a first light source may be positioned on the dividingline D2 and the center L2 of a light emitting point of a second lightsource may be positioned within the fourth region 12D near the dividingline D3 within the assembly accuracy range of the device. Alternatively,as shown in FIG. 8, the center L1 of a light emitting point of a firstlight source may be positioned within the second region 12B and thecenter L2 of a light emitting point of a second light source may bepositioned within the third region 12C within the assembly accuracyrange of the device. Alternatively, as shown in FIG. 9, the center L1 ofa light emitting point of a first light source may be positioned withinthe first region 12A near the dividing line D1 and the center L2 of alight emitting point of a second light source may be positioned withinthe fourth region 12D near the dividing line D3 within the assemblyaccuracy range of the device.

The number of light sources is not limited to two, and three or morelight sources may be provided. For example, in the case where threelight sources are provided, as shown in FIG. 10, the center of a lightemitting point of at least one light source is positioned within thesecond region 12B or the third region 12C within the assembly accuracyrange of the device and the center of a light emitting point of theremainder of the light sources need not be specifically limited.

In this embodiment, the entire diffraction grating 12 is divided intothe first region 12A, the second region 12B, the third region 12C, andthe fourth region 12D. However, a region within an effective beamdiameter range determined by the aperture diameter of the objective lensin the diffraction grating 12 need only be divided into the first tofourth regions, and the region outside the effective beam diameter rangemay have a different dividing state. For example, as shown in FIG. 11,the second region 12B and the second region 13B may not be formed in theregion outside the effective beam diameter range.

In the example shown in this embodiment, the phase difference betweenthe periodic structures of the first region 12A and the periodicstructure of the second region 12B is 90 degrees and the phasedifference between the periodic structure of the fourth region 12D andthe periodic structure of the third region 12C is 90 degrees. However,the phase difference between the first region 12A and the second region12B and the phase difference between the third region 12C and the fourthregion 12D may be any value as long as the phase difference between thefirst region 12A and the fourth region 12D is substantially 180 degreesand the phase difference between the second region 12B and the thirdregion 12C is substantially 180 degrees. In view of the manufacturingerror of the periodic structure of the diffraction grating 12, however,the phase difference between the first region 12A and the second region12B and the phase difference between the fourth region 12D and the thirdregion 12C are preferably in the range of 10 degrees and 350 degrees,and more preferably in the range of 70 degrees and 290 degrees.

FIG. 12 shows an example in which the phase difference between the firstregion 12A and the fourth region 12D is substantially 180 degrees, thephase difference between the second region 12B and the third region 12Cis substantially 180 degrees, and the phase difference between the firstregion 12A and the second region 12B and the phase difference betweenthe fourth region 12D and the third region 12C are substantially 45degrees. FIG. 13 shows an example in which the phase difference betweenthe periodic structure of the first region 12A and the periodicstructure of the fourth region 12D is substantially 180 degrees, thephase difference between the periodic structure of the second region 12Band the periodic structure of the third region 12C is substantially 180degrees, and the phase difference between the first region 12A and thesecond region 12B and the phase difference between the fourth region 12Dand the third region 12C are substantially 180 degrees. The same effectsas those of the diffraction grating of FIG. 3 can be obtained in thesecases as well.

FIGS. 14A through 14C show relations between the shift amount of theobjective lens 19 and the DPP signal amplitude in the case where aDVD-RAM is used as the optical information recording medium 51. FIGS.14A through 14C show examples in which the phase difference between thefirst region 12A and the second region 12B is 0 degrees, 90 degrees, and180 degrees, respectively. Note that, in FIGS. 14A through 14C, theordinate shows the DPP signal amplitude normalized assuming that the DPPsignal amplification value is 100% when the shift amount of theobjective lens 19 is 0 μm. The phase difference between the periodicstructure of the first region 12A and the periodic structure of thefourth region 12D is substantially 180 degrees and the phase differencebetween the periodic structure of the second region 12B and the periodicstructure of the third region 12C is substantially 180 degrees.

As shown in FIG. 14, in the case where the phase difference between theperiodic structure of the first region 12A and the periodic structure ofthe second region 12B is 0 degrees, the DPP signal amplitude changesmore with shifting of the objective lens than in the case where thephase difference between the first region 12A and the second region 12Bis 180 degrees.

FIG. 15 shows a relation between the phase difference between the firstregion 12A and the second region 12B and the change rate of DPP signalamplitude. The change rate of DPP signal amplitude is shown as a ratioof the DPP signal amplitude obtained when the shift amount of theobjective lens is 300 μm to the DPP signal amplitude obtained when theshift amount of the objective lens is 0 μm.

As shown in FIG. 15, the change rate of DPP signal amplitude is closestto 100% when the phase difference between the periodic structure of thefirst region 12A and the periodic structure of the second region 12B is180 degrees. The change rate of DPP signal amplitude decreases as thephase difference gets closer to 0 degrees or 360 degrees.

It is preferable in the optical pickup that the DPP signal amplitude isconstant even if the objective lens is shifted. It is therefore morepreferable that the change rate of DPP signal amplitude is closer to100%. The phase difference between the periodic structure of the firstregion 12A and the periodic structure of the second region 12B cantherefore be in the range of 10 degrees to 350 degrees. For more uniformDPP signal amplitude, however, it is preferable that the phasedifference between the first region 12A and the second region 12B is inthe range of 70 degrees to 290 degrees.

In order to reduce the change rate of DPP signal amplitude, it ispreferable that a light beam that is effectively used includes a portionthat has passed through the first region 12A of the diffraction grating12, a portion that has passed through the second region 12B, a portionthat has passed through the third region 12C, and a portion that haspassed through the fourth portion 12D. In other words, it is preferablethat a region of the diffraction grating 12 on which a range of theemitted light beam corresponding to the effective beam diameterdetermined by the aperture diameter of the objective lens 19 is incidentincludes the first region 12A through the fourth region 12D.

More specifically, in the case where the light source is a DVD-typelight source, it is preferable that the sum of the width W1 of thesecond region 12B and the width W2 of the third region 12C of thediffraction grating 12, that is, the width (W1+W2), is in the range of10% to 40% of the effective beam diameter determined by the aperturediameter of the objective lens 19. The effects obtained by thisstructure will now be described.

First, description will be given to the case where the width (W1+W2) issmaller than 10% of the effective beam diameter determined by theaperture diameter of the objective lens 19. FIG. 16 shows a relationbetween the change rate of DPP signal amplitude and the ratio of thewidth (W1+W2) to the effective beam diameter determined by the aperturediameter of the objective lens 19 in a DVD-RAM. Note that the changerate of DPP signal amplitude is shown as a ratio of the DPP signalamplitude obtained when the shift amount of the objective lens is 300 μmto the DPP signal amplitude obtained when the shift amount of theobjective lens is 0 μm. As shown in FIG. 16, when the width (W1+W2) issmaller than 10% of the effective beam diameter determined by theaperture diameter of the objective lens 19, the change rate of DPPsignal amplitude is smaller than 66%. The DPP signal amplitude thusdecreases by 34% or more of the DPP signal amplitude obtained when theshift amount of the objective lens is 0 μm. Such a large change in DPPsignal amplitude is not preferable.

Next, description will be given to the case where the width (W1+W2) islarger than 40% of the effective beam diameter determined by theaperture diameter of the objective lens 19. FIG. 17 shows a relationbetween the sum (SPP amplitude) of signal amplitude of the push-pullsignal SPP1 and signal amplitude of the push-pull signal SPP2 and theratio of the width (W1+W2) to the effective beam diameter determined bythe aperture diameter of the objective lens 19. As shown in FIG. 17,when the width (W1+W2) is larger than 40% of the effective beam diameterdetermined by the aperture diameter of the objective lens 19, the signalamplitude becomes closer to 0. Since the absolute value of the signalamplitude is reduced, characteristics of the optical pickup device aredegraded.

It is therefore preferable that the width (W1+W2) is in the range of 10%to 40% of the effective beam diameter determined by the aperturediameter of the objective lens.

When two light sources of DVD-type and CD-type are used, it ispreferable that the width (W1+W2), that is, the sum of the width W1 ofthe second region 12B and the width W2 of the third region 12C of thediffraction grating 12, is in the range of 10% to 35% of the DVDeffective beam diameter determined by the aperture diameter of theobjective lens 19.

As has been described above, the optical pickup device of thisembodiment can be used for various optical information recording mediahaving different guide groove pitches and achieves tracking error signaldetection that enables more stable recording and playback. In otherwords, the optical pickup device of this embodiment can implement sizereduction, simplification, cost reduction, improved efficiency, and thelike in DVD- and CD-type recording devices and playback devices.Moreover, the optical pickup device of this embodiment is very useful asan optical pickup device having a signal detection function such as aplayback signal, a recording signal, and various servo signals which areused in an optical head device that serves as a main part of an opticalinformation processor for performing processing, such as recording,playback, and erasure of information, on an optical informationrecording medium such as an optical disc.

INDUSTRIAL APPLICABILITY

The invention can implement an optical pickup device for performingstable tracking error detection on a plurality of optical informationrecording media having different guide groove pitches while maintainingthe advantages of the in-line DPP method. The optical pickup device ofthe invention is useful as, for example, an optical pickup device thatis used in an optical information processor for performing processingsuch as recording of information onto an optical information recordingmedium and playback or erasure of information recorded on an opticalinformation recording medium.

1-14. (canceled)
 15. An optical pickup device for recording informationonto an optical information recording medium and reading and erasinginformation recorded on the optical information recording medium,comprising: a light source; a diffraction grating for separating a lightbeam emitted from the light source into at least three light beams; anda photodetector for receiving the separated light beams reflected fromthe optical information recording medium, wherein the diffractiongrating is divided into a first region, a second region, a third region,and a fourth region having periodic structures of different phases bydividing lines extending in a direction parallel to a tangentialdirection of tracks of the optical information recording medium, thesecond region and the third region are located between the first regionand the fourth region sequentially from the first region side, theperiodic structure of the second region has a phase difference ofapproximately 180 degrees from the periodic structure of the thirdregion, and the periodic structure of the first region has a phasedifference of approximately 180 degrees from the periodic structure ofthe fourth region.
 16. The optical pickup device according to claim 15,wherein a +1^(st) order diffracted light beam that has passed throughthe first region has a phase difference of 80 degrees to 100 degreesfrom a +1^(st) order diffracted light beam that has passed through thesecond region.
 17. The optical pickup device according to claim 15,wherein a +1^(st) order diffracted light beam that has passed throughthe first region has a phase difference of 35 degrees to 55 degrees froma +1^(st) order diffracted light beam that has passed through the secondregion.
 18. The optical pickup device according to claim 15, wherein adistance between the dividing line dividing the first region and thesecond region from each other and the dividing line dividing the secondregion and the third region from each other is equal to a distancebetween the dividing line dividing the second region and the thirdregion from each other and the dividing line dividing the third regionand the fourth region from each other.
 19. The optical pickup deviceaccording to claim 15, wherein a plurality of guide grooves areperiodically formed on a recording surface of the optical informationrecording medium, and each of the light beams is converged on one of theplurality of guide grooves.
 20. The optical pickup device according toclaim 15, further comprising an arithmetic processing circuit fordetecting a tracking error signal by a differential push-pull methodbased on an output signal of the photodetector.
 21. The optical pickupdevice according to claim 15, wherein the photodetector includes atleast three light receiving elements respectively corresponding to thereflected light beams, and each of the light receiving elements isdivided into a plurality of light receiving regions.
 22. The opticalpickup device according to claim 15, wherein a center of the light beamemitted from the light source is positioned in the second region or thethird region.
 23. The optical pickup device according to claim 15,further comprising an objective lens for converging the at least threelight beams onto a recording surface of the optical informationrecording medium as independent convergence spots, wherein a region ofthe diffraction grating on which a range of the emitted light beamcorresponding to an effective beam diameter determined by an aperturediameter of the objective lens is incident includes the first region,the second region, the third region, and the fourth region.
 24. Theoptical pickup device according to claim 23, wherein a sum of a width ofthe second region and a width of the third region is in a range of 10%to 40% of the effective beam diameter.